Flavonoids have been shown to have a wide range of biological and pharmacological activities in in vitro studies. Examples include anti-allergic, anti-inflammatory, antioxidant, anti-microbial, antibacterial, antifungal, antiviral, anti-cancer, and anti-diarrheal activities. Over the past decades, for instance green tea catechins have received significant attention as protective agents against coronary heart diseases and cancers (Bushman, J. L. Green tea and cancer in humans: a review of the literature. Nutr. Cancer 31:151-9; 1998 and Zaveri, N. T. Green tea and its polyphenolic catechins: medicinal uses in cancer and noncancer applications. Life Sci. 78:2073-80; 2006). Green tea catechins belong to the group of flavonoids.
Particularly, (−)-epigallocatechin-3-gallate (EGCG) is the most abundant catechin in green tea and has been extensively studied because of its strong antioxidant and radical scavenging activity (Wang Y.; Ho C.-T. Polyphenolic Chemistry of Tea and Coffee: A Century of Progress. J Agric Food Chem 57: 8109-8114; 2009).
In spite of these desirable properties, clinical use of flavonoids, such as EGCG, has been restricted by their poor stability and limited oral bioavailability. For example, EGCG is unstable at physiological temperature and pH, and readily decomposed with a half-life of less than 30 min. Moreover, the oral bioavailability of EGCG is poor because of its rapid hydrolysis in gastric fluid and metabolic degradation in the gastrointestinal tract. To overcome the limitations, many efforts have been devoted to chemical modification of flavonoids to enhance their stability, bioavailability and biological activities.
Recently, EGCG has been covalently modified with various types of thiol-containing compounds, such as cysteine, glutathione, and proteins, but it is quite challenging to synthesize complex thiol conjugates of EGCG in a controlled manner. One important reason is chemical instability of EGCG. For instance, EGCG readily undergoes autoxidation in neutral and alkaline solution, resulting in dimerization and decomposition.
Moreover, epimerization of EGCG to (−)-gallocatechin gallate (GCG) tends to increase if the autoxidation of EGCG is inhibited by adding superoxide dismutase or by flushing with nitrogen gas. Covalent attachment of thiols to EGCG can also be hampered, by hydrogen peroxide (H2O2) generated during the autoxidation process. Since H2O2 oxidizes free thiol groups to disulphide bonds or sulfenic acids (R—SOH), it can decrease the concentration of free thiol groups that participates in thiol-EGCG conjugation. This leads to low yields and undesired, unreactive disulphide or sulfenic acid by-products in the reactions to make thiol conjugates. A complicated purification step is then necessary to obtain the desired conjugates in high yield and purity.
There is therefore a need to find an effective way to modify flavonoids, such as EGCG, with thiol-containing compounds in a highly selective manner.
Furthermore, there is currently also no way to make stable polymer-flavonoid conjugates. There is however a need to create such conjugates in order to improve the poor stability of the flavonoids and make them available in biomedical applications, such as gels.